Lithium-ion batteries are part of a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and from the positive electrode to the negative electrode when charging.
There are various types of lithium-ion battery. The anode comprises generally carbon and the cathode comprises a lithium compound. The anode and the cathode are separated by a separator made from a porous polymer, such as a micro-perforated plastic sheet, which allows ions to pass through. The anode, cathode and separator are immersed in an electrolyte.
Lithium-ion batteries are classified according to the cathode material.
Once the lithium-ion battery is assembled, before the battery is suitable to be used, the lithium-ion battery may be put through at least one precisely controlled charge/discharge cycle to activate the working material. This step is called the formation process. This formation process provides the initial full charge of the battery.
During the formation process, a solid electrolyte interface (SEI) is formed on the anode. The SEI formation is important for the lifetime of the lithium-ion battery or cell.
Methods for initial charging, i.e., for the formation process, of a lithium-ion battery have been proposed.
Typically, the battery is charged at a constant charge rate. The charge rate is also expressed as a C-rate, which represents a charge or a discharge rate equal to the capacity of a battery in one hour. It has been found that the SEI is best formed at small C-rate, which means that the initial charging is performed over an extended period of time. Indeed, fully charging a battery at a C-rate equal to C/5 would take approximately five hours. The battery is charged at a small C-rate up to the fully charged voltage of the battery in order for the SEI to form on the carbon anode during the first charge and then the battery is held constant at the fully charged voltage until the current drops below a threshold. The battery is then left to rest for two hours and is discharged at a small C-rate to a pre-set voltage, i.e., the discharge cut-off voltage. This formation process may be cycled at least once.
In order to reduce the manufacturing time of lithium-ion batteries, so-called dynamic forming processes have been proposed. In such processes, the battery is charged at a small C-rate up to the end of the SEI layer formation on the anode, corresponding to a threshold voltage value, and then, a large C-rate is used to charge the battery up to the fully charged voltage. For example US 2015/060290 discloses such a formation protocol which still involves at least charging the battery up to the fully charged voltage at least twice, and resting the cell for two hours between each charge/discharge of the cell, the total duration of the dynamic formation process being greater than forty hours. Moreover, the voltage up to which the cell needs to be charged at a small C-rate may only be determined by carrying out tests for each new type or category of cell to be produced. Indeed, this threshold voltage value varies from one type of cell to another.
Additives have also been added to the electrolyte to improve the formation of the SEI and therefore enhancing the anode stability.